The present invention relates to optical communication systems, and more particularly, to an apparatus having one or more narrow band, selectable output wavelengths.
Communication transmission systems based on the modulation of a light signal provide substantially higher bandwidth than systems based on metallic conductors. For example, fiber optic cables are replacing coaxial cable in applications requiring high bandwidth. The capacity of an optical fiber may be further enhanced by multiplexing a number of communication channels on the same fiber utilizing light sources of different wavelengths. In such an arrangement, a number of closely spaced optical channels are transmitted on a single fiber. Each channel carries a separate data stream. Since the wavelengths of the carriers are different, the data streams do not interfere with each other and can simultaneously propagate along a single optical fiber.
The maximum number of channels is typically determined by the width of the individual channels and the maximum range of wavelengths that can be amplified by repeaters on the fiber. The channel widths and interchannel gaps are determined by the width of the spectrum generated by the light source that provides the carriers for the channels. The distance between individual channels is ultimately set by the width of the modulated spectrum on each of the optical carriers in the individual channels.
In many systems some form of gain element must be utilized to amplify the light signals in the fiber in order to compensate for transmission losses. Optical amplifiers are utilized for this function. To minimize distortions, the gain as a function of wavelength for these amplifiers needs to be constant. This condition will only be satisfied for some limited portion of the optical spectrum for any given amplifier design. Accordingly, there is a maximum number of channels imposed by the amplifiers for any given channel spacing. Hence, narrow band sources and filters are needed.
Typically, a narrow band light source is needed for each channel. Data is transmitted on any given channel by modulating the output of the light source for that channel and then introducing the modulated light into the optical fiber. Ideally, the light source should be capable of generating the carrier for any channel, the precise wavelength generated at any given time being selectable by a signal applied to the light source. Light sources having fixed output wavelengths and sufficiently narrow wavelength spreads are known to the art. For example, U.S. Pat. No. 5,268,910 describes a superluminescent optical source for generating an optical carrier from a doped fiber.
In principle, a number of such sources may be combined to provide the requisite WDM signal. However, such a compound source is cumbersome and expensive. Accordingly, it would be advantageous to provide a wavelength selectable source that is less expensive for driving a selected channel in a wavelength division multiplexed (WDM) optical transmission system.
Broadly, it is the object of the present invention to provide an improved filter arrangement for use in optical communication systems.
It is a further object of the present invention to provide a filter arrangement in which the selected wavelength can be controlled by an external signal applied to the filter.
It is a still further object of the present invention to provide a filter arrangement which may be utilized to provide a light source having a narrow band output with a center frequency that is selectable over a range of wavelengths.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a filter for generating an output light signal having one or more output spectral lines selected from a plurality of potential spectral lines contained in an input light signal. The filter includes a channel filter for filtering the input light signal to create a notched light signal having a spectrum devoid of light at a first wavelength. The filter also includes a variable wavelength reflector for reflecting light having a wavelength equal to a reflection wavelength, the reflection wavelength being selectable from the first wavelength and a second wavelength equal to the wavelength of one of the spectral lines. The output of the channel filter is coupled to the input of the variable wavelength reflector. The output of the variable wavelength reflector includes the output light signal. The channel filter can be constructed using a number of known optical components such as fiber Bragg reflectors, array waveguide filters, Mach Zehnder filters, or absorption filters. The variable frequency reflector is preferably constructed from one or more variable frequency fiber Bragg reflectors.